1. Field of the Invention
The present invention relates to a polishing sheet and a manufacturing method of an elastic plastic foam sheet for the polishing sheet, and in particular relates to a polishing sheet having an elastic plastic foam sheet which is of use to precision polishing work and a manufacturing method of an elastic plastic foam sheet for the polishing sheet.
2. Description of Related Art
Conventionally, a polishing sheet is used to perform flattening work on a material required for flatness at a high accuracy, such as an optical material such as a lens, a plane parallel plate, a reflecting mirror or the like, a base plate for a hard disc, a silicon wafer, a liquid crystal glass or the like. For example, as a polishing sheet used for accurate flattening work, e.g. for the silicon wafer or the like, a polishing sheet of a nonwoven type obtained by coagulating a nonwoven fabric after the nonwoven sheet is impregnated with a resin solution is known (refer to JP05-8178A publication). However, since the polishing sheet of this type has flexibility, there are drawbacks in that a polishing efficiency is low and a roll-off, i.e., peripheral edge portions of a material to be polished are polished more than a central portion thereof, occurs easily. In order to improve the polishing efficiency, there has been disclosed a polishing sheet having an independent foam structure obtained by hollow fine spherical bodies or the like in a plastic material with a high hardness (for example, refer to JP08-500622A publication corresponding to WO94/04599). However, because the polishing sheet of this type has a high hardness, for example, when the sheet is used in polishing work for an aluminum base plate for a hard disc, there is a drawback in that defects may occur at a surface of the aluminum base plate.
On the other hand, a polishing sheet obtained by pasting a film with a high hardness or the like to an elastic plastic foam of a suede type with a foam structure produced by wet film forming process has been disclosed (for example, refer to JP10-249709A publication). An elastic plastic foam sheet is generally produced by the wet film forming process, and relatively large cells extending in a direction approximately perpendicular to a surface of the polishing sheet are formed in an interior of the elastic plastic foam sheet. A foaming surface layer (a skin layer), whose thickness is approximately several micro meters and whose surface is flat, is formed at the surface of the polishing sheet. The skin layer is removed by conducting buffing treatment in order to improve flatness of the surface of the polishing sheet, and such a polishing sheet has a honeycomb shaped cell structure that relatively large cells are opened at a surface of a polishing layer. The surface for contacting a material to be polished of the polishing sheet having such a honeycomb shaped cell structure is formed uneven. A polishing liquid is reserved by the opened relatively large cells, and the material to be polished is polished by pressing the reserved polishing liquid at a time of polishing the material. Since foreign substances such as sludge (polishing shavings) and the like are accommodated inside the large cells, scratches (polishing defects) and the like which occur at the surface of the material to be polished can be prevented.
Further, a method for polishing the surface of the material to be polished by a non-buffing type polishing sheet which retains the flat skin layer and which do not have the opened relatively large cells, by giving no buffing treatment to the surface of the above-stated elastic plastic foam sheet, has been disclosed. This method can improve waviness which is one of measurement items for evaluating flatness to the surface of the material to be polished (for example, refer to JP2001-62704A publication corresponding to U.S. Pat. No. 6,439,965). At present, the method for polishing the surface of the material to be polished by this non-buffing type polishing sheet is mainly used in secondary polishing for the aluminum base plate for the hard disc.
Thus, the method disclosed in the JP2001-62704A publication is a method for polishing the material to be polished by retaining a flat face (flatness) of the surface of the skin layer, and which can improve the waviness of the material to be polished. However, since a diameter of the opened relatively large cells changes due to abrasion at the surface of the polishing sheet and polishing performance (waviness of the surface, polishing rate) is gradually lowered, it has a disadvantage in a life span of the polishing sheet. Especially, in polishing for the aluminum base plate or the like for the hard disc having a large capacity, since lowering in the waviness at the surface causes performance dropping in the hard disc, it is required to improve both the waviness at the surface of the aluminum base plate or the like and the life span of the polishing sheet.